Embodiments of the present invention relate to radial rotating machines, such as centrifugal compressors or single stage fluid expanders.
In general, a radial rotating machine may be a rotating machine for processing a fluid flow, the fluid flow being forced to flow radially at least along part of the flow path.
A radial rotating machine is a rotating machine for processing a fluid flow, in which the fluid flow occurs radially at least along part of the flow path. The radial rotating machine may be for instance a centrifugal compressor.
Centrifugal compressors or single stage expanders are radial rotating machines: they comprise bladed impeller wheels, which are designed to force the fluid flow radially away from the axis of the rotating machine.
These impeller wheels are subjected to axial forces which may be of two types: so-called static axial forces, which are generated by the difference in fluid pressure between the upstream side and the downstream side of the wheel, and so-called dynamic axial forces, which are a result of the momentum change imposed to the fluids, flowing in axially into the impeller wheel, and coming out radially out of the wheel.
These axial forces are usually partly balanced by balance drum systems, and partly balanced by axial thrust bearings, for instance by oil bearings.
In said balance drum systems, at least a balance drum part is assembled around the same shaft as the impeller wheel. The balance drum part comprises two radially extending surfaces, facing opposite axial directions, and subjected to different fluid pressures.
These balance drum systems usually are tuned to counterbalance for static axial forces.
According to their design, balance drum systems can sometimes also counterbalance for part of the dynamic axial forces. The remainder of axial forces then has to be counterbalanced with axial thrust bearings. Axial thrust bearings may be of different types. Oil bearings are capable of withstanding high loads, but they have to be fed with the lubricating oils, which may be a hindrance in subsea applications, because of a lack of accessibility of the system, or in medical applications, where contamination by oil cannot be tolerated.
Depending on the maximum axial forces that the thrust bearing can withstand, and depending on the proportion of axial forces not counterbalanced by the balance drum, the fluid throughput of the machine has to be limited, to a value generally lower than the maximum throughput imposed by the other parameters of the radial rotating machine.